Microwaveable bags or packages, particularly those adapted for the microwave cooking or browning of food products, are generally made from a composite that includes several layers of flexible, microwave-transparent sheet material. In general, the outer layer of such a package is paper or cardboard and the inner layer is a liner of polymer film to which can be applied a microwave absorptive layer such as a thin metallic layer to form a metallized film. The film is adapted to conduct thermal energy as heat upon interaction with incident microwave energy to provide a means for browning or crispinq the food product contained within the package. Such microwave absorptive materials are typically referred to as "susceptors".
In order to properly brown or crisp foods, it is desirable to have a susceptor packaging material which supplies enough heat energy to the surface of the food so that the surface can be properly browned or crisped in a time that is sufficiently short so as to prevent the interior of the food from becoming overcooked.
Typically, susceptor containing microwave packaging is formed of a number of sheets of material or plies which are joined together into a composite using an adhesive. The adhesive typically is subjected to temperatures on the order of about 400.degree. F. to about 500.degree. F. during .microwave cooking. Currently, laminating adhesives for use in microwaveable susceptors and packaging products are water-based adhesives designed to replace prior art adhesive polymers such as blends of ethylene vinyl acetate emulsion, vinyl acetate emulsion and a soft ethylene vinyl acetate acrylic latex. See for example U.S. Pat. No. 4,911,960 (Mudqe et. al.), and references cited therein. These prior art adhesive compositions suffer from several disadvantages.
In packaging materials containing adhesives, the adhesive bond between the plies can fail at elevated temperatures, thereby weakening the package and often causing a rupture during use in the microwave oven. Also, during the food cooking process, the adhesive material may volatilize, subjecting the food to solvents and other chemicals which can both compromise the flavor of the food and possibly deposit toxic substances thereon. Further, the polymer film often cracks, thus allowing volatiles to reach the food. There is also an additional processing cost associated with these adhesives and the added cost of lamination.